1. Field of the Invention
The present invention relates to peripheral devices for a computer such as a graphic printer or the like of which the operation is controlled by a command signal from a computer.
2. Description of the Prior Art
When an image formed by a computer is printed out by a printer, if an image is formed on the basis of predetermined image forming data by, for example, a computer, such a data is transmitted to a printer and this printer provides an image equivalent to that of the computer and then prints the same. According to this conventional technique, data can be transmitted from the computer to the printer smoothly in a short period of time so that the image can be printed out while minimizing the influence upon other processings of the computer.
In this arrangement, however, the printer must be provided with an image forming program equivalent to that of the computer. Thus, the computer and the printer must correspond to in a one-to-one relation, losing a general-purpose characteristic of the printer.
Alternatively, it may be considered that data such as hue, brightness level and so on are formed at every picture element of an image formed by a computer and then the data are transmitted to a printer to print the image on the basis of the above-mentioned data. According to this method, however, it takes a lot of time for the computer to form the data to be transmitted. This hinders the other processings to be done by the computer and also this makes the printing time very long.
So far various apparatus have been proposed to print out a video signal derived from so-called video apparatus. Meanwhile, the computer supplies a video signal such as three primary color signals to a display apparatus for displaying, for example, an image.
In view of the above-mentioned aspect, the inventor of the present application has proposed the following technique for deriving and printing out a video signal supplied from the computer to the display apparatus or the like. This previously-proposed technique will be described with reference to FIG. 1.
Referring to FIG. 1, there are provided a computer 100 and a display apparatus 300, respectively. When a video signal generated in the computer 100 is displayed on the display apparatus 300, the video signal supplied to the display apparatus 300 is supplied through a video interface 6 to a graphic printer 200. Thus, this video signal is printed out on a printing paper 500. Input and output data or the like of desired control signals from the computer 100 are supplied through an existing interface 1 to the printer 200.
In this case, however, the video signal from the computer 100 is different from that of the general video apparatus in such a fashion that a signal to be formed is changed stepwise at every picture element. Thus, when such a video signal is printed out, a sampling phase at the printer must exactly coincide with the phase of the video signal from the computer. Should both the phases be displaced, there may be a risk that the printing would be displaced at each picture element or become impossible.
Meanwhile, in accordance with the standards of the general computers, the phase of the sampling clock is arranged to have a predetermined relationship relative to a horizontal synchronizing signal of, for example, the video signal. However, when a lot of compatible terminals are provided for the above-mentioned original computer, each of such compatible terminals is intended only to effect the display on, for example, the display apparatus so that the above-mentioned sampling phase is not always coincident with the computer. Accordingly, if the printer is designed so as to make the sampling phase comply with the standards of the original computer, when connected with the above-mentioned compatible terminal, the printer causes the picture elements of the printing to be displaced because of phase displacement and the printing becomes impossible.
Further, when the operation of peripheral devices such as a graphic printer or the like is controlled on the basis of a command from a host computer, a given code is generally set in the peripheral device side. Thus, when this code is instructed as a command data of the software from the host computer, the predetermined operation is effected.
In the above-mentioned apparatus, however, in order to replace a dot printer system with a color graphic printer or the like, if the code set in the color graphic printer coincides with that of the conventional dot printer, there will occur no problem. If they are different, the corresponding software or the like must be rewritten. Thus, when the above-mentioned system is modified, special knowledge for rewriting the software or the like is needed and the modification cannot be carried out without difficulty.
According to the above-mentioned conventional technique, when the video signal from the computer is printed, if the video signal having displaced phase is supplied thereto from the compatible terminals or the like, the picture elements for printing may be displaced or the printing becomes impossible. Also, when the system is modified, the software or the like must be rewritten so that the modification cannot be carried out without difficulty.